In recent years, semiconductor optical amplifiers (SOAs) which vary their gain with drive current are attracting attention as optical amplifiers in optical communication systems.
The SOA is an optical amplifying element made of compound semiconductor such as indium phosphide. The output is not as large as the output of an erbium-doped fiber amplifier (EDFA), which uses erbium-doped fiber as an amplifying medium. With advantages such as small size and wide amplification band, the SOA can be used also as an optical gate element and has been becoming widely used in next-generation photonic systems.
The graph in FIG. 10 represents output levels of the SOA. The vertical axis represents the output level in dBm, and the horizontal axis represents the input level in dBm. The graph demonstrates the output levels varying with the input level, depending on the drive current injected into the SOA.
The output level of the SOA depends on the injected drive current, and if the optical input level is the same, the optical output level increases with increased drive current. There are two regions divided by a certain input level L0. In a region up to the input level L0, the output level increases linearly with increase in input level. In a region beyond the input level L0, the output level stops increasing. The former region is referred to as a non-saturation region, and the latter a saturation region.
The gradient of the line corresponds to gain. The constant gradient in the non-saturation region means that the gain is constant. The gradient in the saturation region becomes smaller than that in the non-saturation region, meaning that the gain decreases.
The graph in FIG. 11 represents the gain of the SOA. With the vertical axis representing the gain in dB and the horizontal axis representing the output level in dBm, the graph represents the gain characteristics of the SOA receiving continuous light. The graph clearly indicates that the amplifier gain drops sharply with increase in the output level in the saturation range.
The SOA has such amplification characteristics in the non-saturation region that the gain is constant while the injected drive current is constant and that increased input level increases the output level. In the saturation region, the gain decreases with increase in input level. This prevents the output level from exceeding a certain level.
For example, if the SOA with an optical input of −10 dBm delivers an amplified optical output of 0 dBm, the gain is 10 dB. With a constant drive current, the 10 dB gain is kept in the non-saturation region, to deliver an optical output of 5 dBm from an optical input of −5 dBm.
In the non-saturation region, the gain is kept constant irrespective of the input level, and the output level becomes proportional to the input level. In the saturation region, however, the gain is not maintained but decreases, and the output level will be saturated (the increased output level reaches a limit).
The diagram in FIG. 12 illustrates the structure of a conventional optical amplifier 50 using the SOA. The optical amplifier 50 includes a variable optical attenuator (VOA) 51, an SOA 52, a monitor unit 53, a VOA control unit 54, and a coupler C. The monitor unit 53 includes a photodiode (PD) 53a and an A-D converter 53b. 
The VOA 51 performs attenuation control (loss level control) for the level of the optical signal input to the optical amplifier 50, in accordance with a control signal. The optical output from the VOA 51 is split into two parts by a coupler C, one part being input to the SOA 52 and the other part being input to the monitor unit 53. The SOA 52 amplifies the optical input in accordance with a drive current specified to provide a desired gain.
In the monitor unit 53, the PD 53a converts the optical input signal to an electric signal. The A-D converter 53b converts the electric signal to a digital signal and outputs it as a monitored value. The VOA control unit 54 adjusts the level of the optical input to the SOA 52 appropriately in accordance with the monitored value, so that the SOA 52 can deliver a desired level of amplified optical output without saturation.
A disclosed conventional optical amplification control technology keeps a constant amplification factor of the SOA through keeping a constant level of optical input power by means of an attenuator (refer to Japanese Laid-open Patent Publication No. 2002-208758 (paragraphs [0004] and [0005], and FIG. 1)). In another disclosed technology, false optical signals are provided on the short wavelength side and the long wavelength side of the wavelength band and multiplexed with the optical signal for optical amplification, and the wavelength characteristics within the wavelength band of the optical signal are controlled in accordance with the monitored results (Japanese Laid-open Patent Publication No. 2000-307552 (paragraphs [0036] and [0037], and FIG. 1)).
In the conventional optical amplifier 50 described above, if the input optical signal is held to a certain level for a long time (the period of level ‘0’ or ‘1’ lasts long), the SOA 52 could deliver an optical output with distorted waveform or could not provide a desired level of output.
FIG. 13 illustrates a single level maintained in different signal formats. In the synchronous optical network/synchronous digital hierarchy (SONET/SDH) signal format, up to 72 consecutive bits in a single frame can be held to ‘0’ or ‘1’.
In the SONET/SDH format, the maximum number of consecutive bits that can be held to the same level within a single frame is predetermined, irrespective of the transmission rate. Since a lower transmission rate means a longer duration of a single pulse, the same level is maintained for a longer period.
In the Ethernet (registered trademark) signal format, the maximum number of consecutive bits that can be held to the same level within a single packet is not specified. However, a gap (corresponding to consecutive ‘0’ bits) between packets is significant in the Ethernet. The minimum packet gap is specified as 12 bytes (96 bits).
A lower transmission rate means a longer duration of a ‘0’ bit, expanding the packet gap between packets n and n+1 and increasing the duration of level ‘0’.
FIGS. 14 to 16 show monitored values depending on unbalanced bits. Being driven at a low rate in comparison with the transmission rate of the optical input signal, the monitor unit 53 cannot monitor the optical power in bits. The monitor unit 53 monitors the average power of varying bits and outputs it as a monitored value. If the bit values are unbalanced, the monitored value will increase or decrease (if there are many ‘0’ bits or ‘1’ bits, the average power will decrease or increase).
FIG. 14 presents the average power of an optical input signal when the mark ratio (probability of occurrence of ‘1’ bits) is about 1/2. If the probabilities of occurrence of ‘1’ bits and ‘0’ bits are about the same, the average power (normal monitored value) is 3 dB lower than the peak level of the optical input signal.
FIG. 15 presents the average power of an optical input signal when the mark ratio is close to 1. If the probability of occurrence of ‘1’ bits is high, the average power approaches the peak level. FIG. 16 presents the average power of an optical input signal when the mark ratio is close to 0. If the probability of occurrence of ‘0’ bits is high, the average power approaches the bottom level.
Suppose that level ‘0’ lasts long. If an optical signal having more ‘0’ bits is input to the VOA 51, the VOA control unit 54 judges from the monitored value (indicating the average power) that the level of the optical input signal has been lowered (see FIG. 16).
Accordingly, attenuation control is applied to the VOA 51 to decrease the optical loss level at the VOA 51, that is, to decrease the amount of attenuation at the VOA 51 (as if the VOA 51 is bypassed to let the input light advance without attenuation).
If an optical signal of level ‘1’ is input while the VOA 51 is set to decrease the amount of attenuation (as if the VOA 51 is bypassed), the SOA 52 would receive an excessive level of light and would operate in the saturation region. This would decrease the gain, saturate the optical output, and distort the output waveform.
FIG. 17 illustrates waveform deterioration caused by lowered gain. The vertical axis represents the SOA output level, and the horizontal axis represents the SOA input level. In the graph presenting the output level varying with the input level, the solid line g1 represents the actual SOA state, where the output is saturated with the input level exceeding L0. The dashed line g2 represents the ideal state, where the output is not saturated with the input level exceeding L0.
If the SOA 52 receives an excessively high level of optical input, the peak of the optical output signal of the SOA 52 is distorted. This would close eye apertures of the eye pattern, making it hard to differentiate between ‘0’ and ‘1’ on the receiving side.
Suppose that level ‘1’ lasts long. If an optical signal having more ‘1’ bits is input to the VOA 51, the VOA control unit 54 judges from the monitored value that the level of the optical input signal has been raised (see FIG. 15).
Accordingly, attenuation control is applied to the VOA 51 to increase the optical loss level at the VOA 51, that is, to increase the amount of attenuation at the VOA 51 (while the VOA 51 is operated to let the input light advance with attenuation).
If normal data (with almost equal numbers of ‘0’ bits and ‘1’ bits) is input to the VOA 51 which is set to increase the amount of attenuation (while the VOA 51 is operated normally), the SOA 52 would receive optical input at a lower level than is needed. Although the low input level will not make a saturation region, amplification to a desired output level will be impossible. The table in FIG. 18 lists the problems of the conventional optical amplifier 50 caused by the continuation of either level.